[Awarded Senn Medal at the Fifty-First Annual Meeting of the American Medical Association, June 6, 1900. Received by the Committee, March, 1, 1900.]

EMBRYOLOGY.Before considering a congenital deformity of the bladder, it will be advisable to review briefly the embryological development of the genito-urinary system, and, of necessity, the embryo as a whole.The ovum, a single cell with a single nucleus, soon after impregnation becomes, by karyokinesis, a mass of cells resembling in shape and appearance a mulberry, hence the name, morula. A clear fluid next accumulates in the center of this mass of cells; at first small, but gradually increasing in amount. Thus, the morula is converted into a globular vesicle, termed the blastodermic vesicle, the wall of which is at first composed of a single layer of cells—ectodermal, but later, a second layer of cells—entodermal—is completed. This double layer of cells is termed the blastoderm. The